Transistor flip-flop circuit



April 19, 1960 w. J. POPPELBAUM 2,933,621

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United States Patent "Of u TRANSISTOR FLIP-FLOP CIRCUIT Wolfgang J. Poppelbaum, Champaign, Ill., assignor to 1 University of Illinois Foundation Application August 2, 1956, Serial No. 601,707

6 Claims. (Cl. 307-885) This invention is concerned generally with the electronic arts, and more particularly with a transistor' cross-coupled ip-op circuit, particularly for use in digital computers.

As is known in the art of digital computers, coupled Hip-flop circuits which have two stable conditions, and an intermediate unstable condition, are used to compute and indicate coded intelligence. 'In a racing register, i.e., a combination of two ip-ops, the rst one setting the second, which in turn sets the rst one back again, the succeeding stage is triggered when the ip-op has gone through fty percent of its swing, through the intermediary of a gate in the nature of a difference amplifier. Obviously, the faster the circuit can change its state, and trigger the succeeding circuit, the faster the computer can work. The fastest tube circuits generally operate no faster than on the order of 50 mns. (50 millimicroseconds or 50x10"9 seconds).

It is an object of this invention to provide a transistor :Elip-op circuit operable at substantially higher speeds than any ip-op circuits heretofore known.

MoreV particularly, it is an object of this invention to provide a ip-op composed of a pair of bistable circuits coupled to and controlling one another, each of the bistable circuits comprising a grounded-base-arnplier controlled by and working into an emitter-follower.

More specifically, it is an objectV of this invention to provide a transistor op-op circuit utilizing emitter-base cross-coupling.

Other and further objects and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanyingv drawing wherein the single igure comprises a schematic wiring diagram of the invention.

The circuit now to be described is generally symmetrical, and for purposes of clarity the symmetrical parts will be identied by similar numerals, those on one side of the circuit having the suiiix a, and those on the other side of the circuit having the suffix b. As is common in circuits of this nature, there are two input connections respectively numbered 10a and 10b. The input 'connections are directly connected to the bases 12a and 12b of a pair of transistors 14a and 14b forming a gate 16. The emitters 18a and 18b are connected in common to aV junction 20 which is connected through `a resistor 22 to an external connection indicated by the arrowhead 24; The resistor 22 preferably is of 2000 ohms, and the external connection represented by the arrowhead 24 is supplied either with plus ten volts, or minus eight volts, respectively to enable or to disable the gate 16so as to render the circuit operative or inoperative in the computer Vor other device in which it is incorporated.. The collectors 26:1 and 2Gb of the. transistors 14a and 14b are respectively connected to common wires or buses 28a and 2812.

Each of the individuai ip-op circuits including the buses 28a and 28h, and the parts hereinafter to be set forth is shown as enclosed within a dotted line box, the

` n Patented Apr. 19, 1960 former being identied by the letter A and the latter by the letter B. Only the parts within the box'A will be described, it being understood that the parts and connect'ions thereof in the box B are identical, as readily can be seen by the use of similar reference numerals, except` for the difference in suixes.

-The bus 28a is connected through a resistor 30a to ai transistors could be upn transistors if the polarities of all voltages are reversed. The base of the transistor 34a is identified by the numeral 36a, and the connections thereof will be set forth hereinafter. The emitter 38a is connected to a junction point 4in1, and this junction point is grounded through a resistor 42a. The junction pointY 40a further is connected to the emitter 44a of a transistor 46a. The collector 48a of the transistor 46a is connected toa fixed D.-C. potential source of -10 volts. The base 50a of the transistor 4e@ is connected to a junction point 52a.

The junction point 52a is connected through a resistor 54a tota ixed source of D.-C. potential of +10 volts. In the-illustrative example, the resistor 54a is of 4700 ohms. The junction 52a further is connected through a resistor 56a to the bus 28a, and arcapacitor 58a is in parallel with the resistor 56a between the junction 52a and the bus 28a. In the illustrative example of the invention, the resistor 56a has a value of 1200 ohms, forming With'the resistor 54a a voltage divider, and the capacitor has a value of .001 microfarad.

It previously has been noted that the connections of the base 36a of the transistor 34a. would be set forth at a latertime. The connection is of extreme importance, and it will be noted'that a wire 60a leads from the base 36a to the junction point 4Gb, whereby the base 36a is coupled to the emitter 38h. Similarly, the wire 60b. couples the base 36h to the emitter 38a. The wire 60a is pr'ovided with an output connection 62a (the wire 60b simi-. larly being provided with an output connection 62b).

Consider now only the circuit arrangement shown within the boX A. The transistor 34a acts essentially as a grounded base amplifier, while the transistor 46a acts as an emitter-follower. The connection of the two cornprises a bistable circuit. The bistable nature of the circuit readilycan be understood from the following reasoning: Suppose that 32a starts conducting, i.e., that the base-collector current increases. Ihe point P thus will become more positive, and this voltage change will be transmitted to the base 50a of the transistor 46a through the divider network including the resistor Stia-and the capacitor 58a. Transistor 46a acts as an emitter-follower, increasing the potential of Q, and therefore producing still more current in transistor 36a. This race only stops when P becomes so positive that transistor 46a cuts oit. Similarly, if the current of the transistor 34a decreases, all of the eiects are opposite to those just described, and the race" only stops when transistor 34a is cut oft". The circuit will be stable in either of these two states, but not at any intermediate point. Thus, when transistor 34a is on, it means that transistor 46a is ofi, and vice versa. The transistors 34b and 46b form a separate ip-op similar to that just described.

' Now consider the bases 36a and Sb of the transistors 34a and 34b. The base 36a is cross-coupled to the emitter 38h, as is the base 36b to the emitter 38a. Accordingly, if transistor 34a is on, then the transistor 34b is off. However, the potential difference between the bases is very small because of the small and nearly constant emitterbase voltage drop in a transistor. Furthermore, it is 3 E easy to see that transistor 34aV and transistor 4Gb (or-transistor 46a and transistor 34h) are on or off together.

Thus, under all circumstances,` one of the base leads of erzland 341': isheld at aflxed potential by the corresponding emitter follower 46a or 4615. From this, and from the fact that the Voltage betweenthe bases 36a and l36b isV negligible, it follows that the base 36a, for-example, is at ,a very nearly constant potential, ie., transistor 38a acts glilie al grounded base or GB-ampliiier. Thus, the comtplete circuit including the parts within the boxes A and B is essentially formed of two independent bistable parts cross-coupled in such a'way that therstates thereof are always opposite. It is to be noted that the voltage divider comprising the resistor 54a and theresistor 56a is calculated in such a way that the emitter-follower 46a`always cuts off before the amplifying transistor 34a isr saturated.

hus, the undesirable statevof paralyzation of a transistor when it becomes saturated is avoided without the necessity ofthe diode limiting rfrequency resortedyto in order to prevent such saturation. Y

The circuit has two stable states, as -will be obvious from the foregoing discussion. For theV circuit values noted, one of these states corresponds to an output voltage of. 2.8 volts at output point 62a, and 3.6 volts at loutput point 62]). In'the other stable state the output voltages are reversed. Either or both of theltwo output terminals can be used to furnish information about which state "the Hip-flop is in to another circuit. The two transistors 14a and 14h forming the gate 16 may be used to change the flip-flop from one state to another if voltages I on the input terminals a and 106 of 2.8 4volts and 3.6 volts respectively, or vice versa, are applied thereto.

The circuitincluding the gate has been used in a racing register (i.e. a combination of two iiip-tiops, the first one setting the second, which in turn sets the first one back again). The next stage is triggered when the flip-flop has gone through tifty percent of its swing, as ycan be seen from the fact that the gater is av difference amplifier. The time taken to reach the fifty percent -point turned out to to be on the order of 2OV mns. This'is believed to be substantially less Vthan any other circuit heretofore known in thegart. It will be noted that the fast operation in partl is brought about by the fact that the' trigger signal from gthe collector side of 34a or 34h is able to reach the emitter thereof by ashort capacitive path. Thisy path is formed byv the. .005,- microfarad capacitor (58a or58b) and the large intrinsic emitter-base capacitance of the conducting emitter-follower 46a or 46h. Accordingly, the settingiof such'a .cross-coupled ipfop takes a very short` time.

lt will beapparent from the foregoing that a ip-op circuit of superior speed characteristics has been provided. Other advantages are provided by the transistors. The smaller physical size makes shorter leads possible than with tube'circuits, and stray capacitances thuswill be lower. Advantages will'be seen in the lower power VconsumptionJo-f the transistors, particularly in connection withy cooling problems. Furthermore, the life expectancy of VtheY transistors is much higher than'that of'tubes.-

Various circuit `values vhave been given by' wayof specific example, and it has been noted that the transistors arieK of the pnp type, althoughY npn type may be employed.

,arsenal 4 The invention lis claimed.A as Y follows:

1. A flip-dop circuit comprising a pair of similar bistable-circuits, eac-h circuit includng two transistors and each circuit comprising a direct-current-coupled in-phase amplitier connected to and supplying an emitter-follower and crosswise inter-connections between the emitters and bases of the voltage in-phase ampliers, input means to supply control signals to said bistable circuits to reverse the states of conduction land non-conduction in the said circuits, kand output terminals to derive output voltages from said bistable circuits. i

2. The circuit claimed inrclaimrl comprising, in addition, parallelly connected capacitor and vresistor means connected between the collector element of each voltagein-phase ampliiier and the base of the thereto connected emitter-follower. n

3. The dip-flop circuit claimed in claim 2 wherein the input comprises a gating VVam-plier including a pair of transistors, and means to bias theemitters of the transistors selectively to produce conduction and non-conduction. Y

4. The. iiip-'fiop circuitV claimed in Vclaim 3 comprising, in'addition, a connection tothe base of each of the pair of transistors-of the gating amplier for controlling the conduction and non-conduction thereof and a connection betweenV the collectcrelements ,df each of @otherV of the transistors of the secondbistable circuit Specically, and yby way of example, the transistors used y are. Philco SBlOO. VThe speciic exampley of the invention, aswell as the vs'pecilic circuit values and transistor types, will be understood as being by'way of illustration only.

- Various changes'willrio doubt occur to those'skilled in and Ythe emitter of the corresponding transistor of the iirst bistablecircuit. Y Y

6. A flip-flop circuit comprising Varpair of similar bistable circuits each including twotransistors, one transistor of each circuitV being a grounded base Vamplifier and the other transistor of each circuit being an emitterfollower, each bistable circuit having normally two stable 'states and a non-stable state of transition betweenV the two stable states, the transistors normally being in oneV of the two stable states, a connection between the base of each grounded base amplifier-transistor andthe emitter `of the other transistor of the other bistableA circuit so that the grounded-base ampliier transistors are in relative opposite states of conduction and non-conduction, a dierence amplifier, `means toconnect the diference amplitier to supply its output to control the conductive statesv in one bistable circuit while concurrently controlling the non-conductive state in the other bistable circuit, input means to supply control voltages to the difference amplitier so-that the output changes produce reversals in the states of conduction and non-conduction in the bistable circuits, and output means connected to each flip-op circuit.

References Cited in the tile of this patent OTHER YREEi-:RENCES ,'Pub. VHI, Junction-Transistor Trigger Circuits, Vb Flood, in Wireless Engineer, Mayl955, pp. 122-130.

Electronics, June 1955, pp. 132-136.

YElectronics for August 1955, pp.- 1172 `and 113. 

